Optical bodies must satisfy, simultaneously, exacting specifications with respect to most of their properties. These specifications include maximum optical homogeneity, high and uniform transmission throughout the range of wavelengths characteristic of the particular material, low internal scatter, and absence of internal stresses. It is difficult enough to grow small, nearly perfect crystals, but to grow large ingots of the same near-perfect quality in the ambience of a mass-production facility, demands an exceptionally compelling attention to processing techniques.
It is known that Stockbarger growth requires generally purer raw materials. Thus, for economic reasons, Kyropoulos growth is preferred for mass production of ingots of alkali metal halides and particularly of potassium bromide. Moreover, Kyropoulos growth is both easier and faster than growth in a Stockbarger furnace. Despite the less purified growth stock used in Kyropoulos growth the ingot conventionally obtained is of generally high quality, exhibiting only slight haze, and generally acceptable levels of silicate absorption at 9.5-11 .mu., nitrate absorption band at 7.2 .mu. , and, sulfate absorption bands at between 8 and 9 .mu. . When especially high quality crystals are desired, for example, crystals of KBr and KCl such as are necessary for maximum transmittance of a beam of high energy radiation, the crystals must be essentially absorption-free especially at 10.6 .mu. where the major silicate band and compensated sulfate bands fall. It is now possible to grow such crystals by either method.
Briefly the Stockbarger method utilizes a dual-zone furnace having separately heated, individually controlled upper and lower zones. The zones are separated by a polished diaphragm or baffle through which a crucible may be controllably lowered on an elevator mechanism. The crucible is typically cylindrical and tapers to a point to form a conical bottom. Highly purified growth stock salt in the crucible is melted in the upper zone, the temperature being less than that required to cause noticeable evaporation. The crucible is then lowered slowly into the lower zone which is maintained at a temperature below the crystallization point, but not so low that the ingot will crack. A crystal is produced in the tip of the conical bottom, and grows upward as the crucible is lowered, until the entire melt forms a macrocrystal, essentially single crystal ingot. It is well known that a certain zone-purification effect inheres to Stockbarger growth and the impurities are concentrated ahead of the growth interface away from the cone of the crucible. With conventional highly purified materials it is possible to limit haze by growth rate or time given to allow the melt to clarify, but absorption bands from melt-soluble impurities, albeit at low levels, cannot be so limited, and are usually present throughout the ingot.
The Kyropoulos method, as improved by numerous workers over the years, utilizes a large cylindrical crucible, closely fitted with resistance coils to control closely the temperature of the contents of the crucible. The crucible may be of platinum or silica, and is filled with a mass of finely divided salt which is heated until the mass melts. More growth stock salt may be added until the melt fills about three-fourths of the volume of the crucible. The temperature is then raised about 100.degree. F above the melting point. A seed crystal (a piece of a single crystal), held in a coolable holder, is inserted into the melt at the center, and rotated slowly as the temperature of the melt is reduced until the interface between the crystal and the melt is supercooled. This causes the seed to grow.
Initially, the seed grows radially; when the diameter becomes slightly less than that of the crucible, the seed may be set in slow vertical motion, if necessary, i.e., pulled, so the crystal growth is a desirable combination of the rate of pulling and the rate of drop in melt level. A proper choice of pulling speed results in a boule or ingot of fairly regular cylindrical shape, with a height roughly equal to its diameter.
Actual operation of Kyropoulos growth furnaces is somewhat more complicated, but it is to the improvement of the basic method outlined above, that this invention is directed. More specifically, there is currently a great emphasis on production of large, near-perfect crystals of alkali metal halides particularly for use as windows for transmittance of high power laser beams without significant absorption. The commercial aspects of producing such crystals dictate that an economically purified growth stock salt be used, despite the impurities present in it. There is an especial need for an economical method for producing such windows in a Kyropoulos furnace, but no such method was known. The process of this invention and the boules possessing a unique crystalline structure, produced therewith, are directed to filling this need.